Different filtering arrangements, which are there to prevent the dimmer causing interference to other systems, and different firing arrangements, which are how the dimmer controls the on/off switching of the thyristors/triacs (the devices which actually contol the 'power'), can all cause varying amounts of leakage.
Actually, thinking about it, the Betapack 1s do have a preset pre-heat adjustment inside. It night be worth getting a competent engineer to check that they don't need adjusting.
My previous statements about leakage still apply though.